In the field of technology for equally coating slurry as a thin film, it has been known a variety of coating methods such as a die coating, a spin coating, a roller coating, etc. However, the method of the spin coating may cause the problem of uneven thickness, i.e., thicker in center area, when the gravity weight of the slurry exceeds over a specific range. On the other hand, the roller coating is more suitable for coating continuously. In general, the die coating may be suitable for coating continuously or discontinuously.
The apparatus for coating die is referred to as a die coating apparatus. Referring to FIG. 5 and FIG. 6, the die coating apparatus generally is provided with a container 101 for storing coating slurry; a pump 102 connected to the container 101 through a first pipe 103; a coating head 105 connected to the pump 102 through a second pipe 104; and a first valve 106 provided between the pump 102 and the coating head 105 through the second pipe 104; a third pipe 107 and a third valve 110 provided between the coating head 105 and the container 101; an elevator mechanism 112, connected to the coating head 105, for controlling the distance between the coating head 105 and the substrate 120; and a second valve 109 connected between the container 101 and the second pipe 104 through a fourth pipe 108. The coating head 105, which is for coating the slurry on a substrate 120, is almost placed horizontally, and thus this kind of coating is generally referred to as a horizontal coating.
To observe the detailed configuration of the coating head 105 of the prior coating apparatus and the coating mechanism, as shown in FIG. 5 and FIG. 6, the interior of the coating head 105 has a discharging concave 1051 and a feed-back concave 1052, wherein the discharging concave 1051 has an discharging pipe 1055 connected to the second pipe 104. The feed-back concave 1052 has a feed-back pipe 1056 connected to the third pipe 107. The radical size of the discharging concave 1051 and the feed-back concave 1052 are the same. Further, the discharging pipe 1055 and the feed-back pipe 1056 are provided in parallel.
The coating steps thereof are described as follows. First, the protective cover (not shown) is covered on the front end P of the coating head 105 to avoid the slurry flowing therefrom. The pump 102 is then started. The first valve 106 and the third valve 110 are opened, and the second valve 109 is closed simultaneously, so as to fulfill the first pipe 103, the second pipe 104 and the third pipe 107 with the coating slurry to thus eliminate the gas bubbles which may be existed within those pipes. Then, the operation of coating is to proceed. The protective cover is removed, and the elevator mechanism 112 is operated to move the coating head 105 toward the substrate 120 within a predetermined distance. The third valve 110 is then closed and the second valve 109 keeps closed. The coating slurry is thus coated on the substrate 120 only through the path in an order of the first pipe 103, the second pipe 104, the first valve 106, and the coating head 105.
Herein as an example, referring to FIGS. 5 and 6, it is illustrating a die coating apparatus that operating coating on a flexible substrate 120, which is longer in length. The flexible substrate 120 is wound around between a discharging shaft R and a feed-back shaft Rw. The flexible substrate 120 is moved upward and downward by the rotation of the discharging shaft R, the feed-back shaft Rw, and a roller R1, R2 provided therebetween. The flexible substrate 120 is thus applied by an appropriate force to allow a flat coating position, i.e., the flat position around the coating head 105, and to have the slurry uniformly coated on the substrate 120 in an operation of continuous coating.
When it is about to stop discharging slurry from the coating head 105, the third valve 110 is kept closed, and the first valve 106 is closed and the second valve 109 becomes open, so as to recycle the slurry through path from the first pipe 103, the second pipe 104, and the fourth pipe 108, to the container 101. At the moment when the first valve 106 is closed, a phenomenon of tiny vacuum happens between the first valve 106 and the coating head 105, and thus it generates a pulling force for the slurry existing in the coating head. Further, in view of micro perspective, a viscous force happens among the slurry, and thus it prevents the slurry from dropping from the coating slit 1057.
However, it is very difficult to control the magnitude of the pulling force and the viscous force. The aperture diameter of the coating slit 1057 is much smaller than that of the discharging concave 1051 and the feed-back concave 1052. Thus, the slurry residual in the coating slit 1057 is not easy to be out of the coating slit 1057, so the residual slurry still exists in the coating slit 1057. Consequently, the slurry still continues flowing from the coating head 105 after the first valve 106 is closed.
On the other hand, as can be observed from the above content, the slurry in the coating head 105 and a portion of the third pipe 107 proximate to the coating head 105 is in a still state after the first valve 106 is closed. The slurry is allowed to flow only if overcoming a static frictional force existing in the coating head 105 and the third pipe 107. It is well-known that a static frictional force is always larger than a dynamic frictional force. In other words, an applied force overcoming a static frictional force will be too large in view of the dynamic frictional force. Accordingly, in the prior art, slurry which starts to be pushed by the pump 102 is too much, and it causes the slurry to be pushed thereafter is disadvantageously too big in size. That is, the starting portion and the ending portion of the coating pattern coated on the substrate 120 are in shape of convex as shown in FIG. 5 and FIG. 6, which are not in even and flat shape as required.
Hereinafter, we consider another coating method naming vertical coating. When the substrate is not flexible and is not wound for being horizontally coated with slurry as shown in FIG. 5 and FIG. 6, the substrate 120, positioned below the coating head 105, moves toward a specific direction. The coating head 105 coats the slurry on the substrate 120 in a perpendicular manner. In considering the period that the coating head 105 starts discharging slurry and stops discharging slurry, or the period that the slurry is re-discharging from the state of stop discharging slurry, it is found the viscous force within the slurry itself is comparative small since the slurry contains a large percentage in a range about within 40% to 70% of solid material. Moreover, since the weight of slurry causes a major affect, the phenomenon that the slurry residual within the coating slit 1057 becomes more serious. Thus, the phenomenon as described that “slurry which starts to be pushed by the pump 102 is too much, and it causes the slurry to be pushed thereafter is disadvantageously too big in size” becomes more serious. More, the residual slurry is unable to be restore and fed back completely, which is still survived in the coating head 105, as shown in FIG. 6. The residual slurry becomes very easy to drop on the substrate 120. It causes the drawbacks that the patterns on the substrate 120 are not expected, or that a thickness of a specific portion on the substrate 120 is much thicker than requested. The above drawbacks both cause the reasons of generating a flaw substrate. The removal of the slurry residual on the front end of the coating head 105 also further causes a waste of slurry. For the sake of the above, a coating apparatus with the above structure is generally used for horizontal coating. Furthermore, the drawbacks can only be overcome by cutting the uneven portion or the portion which belongs to a flaw pattern.
Besides, regardless the continuation coating, discontinuation coating, or coating on a wound substrate with a longer size, although the distance between the coating head and the substrate is adjusted by applying the elevator mechanism, or the thickness of slurry is controlled by adjusting the forward speed of the substrate by utilizing the rotating device such as rotating shaft R, however, it only can perform the operation of coating with a simple rectangular shape. In other words, the goal that generates a variety of shapes could not be achieved.
Thus, it becomes an important issue about how to remove the slurry from the coating slit 1057 of the coating head 105, or even remove the slurry from the discharging concave 1051 when stopping coating. It also becomes an issue about how to recycle the slurry residual in the coating head 105 in view of situation it provides an excellent expected pattern with uniform and even thickness, and prevents the waste in material of slurry and substrate.